Core-shell structured mesoporous silica: A new immobilized strategy for rhodium catalyzed asymmetric transfer hydrogenation

Article abstract of DOI:10.1039/c2cc33512c

A core-shell structured heterogeneous rhodium catalyst exhibited excellent catalytic activity and enantioselectivity in asymmetric transfer hydrogenation of aromatic ketones in aqueous medium, which could be recovered easily and used repetitively twelve times without affecting obviously its enantioselectivity.

Full text of DOI:10.1039/c2cc33512c

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Cite this: Chem. Commun., 2012, 48, 7874–7876
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COMMUNICATION
Core–shell structured mesoporous silica: a new immobilized strategy for
rhodium catalyzed asymmetric transfer hydrogenationw
Huaisheng Zhang, Ronghua Jin, Hui Yao, Shuang Tang, Jinglan Zhuang, Guohua Liu* and
Hexing Li*
Received 15th May 2012, Accepted 19th June 2012
DOI: 10.1039/c2cc33512c
A core–shell structured heterogeneous rhodium catalyst exhibited
excellent catalytic activity and enantioselectivity in asymmetric
transfer hydrogenation of aromatic ketones in aqueous medium,
which could be recovered easily and used repetitively twelve times
without affecting obviously its enantioselectivity.
as a structure-directed template possesses a potential function
of phase transfer. Thus, use of residual CTAC within CSSMSS
as a phase transfer catalyst in a biphasic catalysis system can
enhance catalytic efficiency significantly.
We were interested in the mesoporous silica-supported catalysts,
in particular, in an effort to achieve the high recyclability of
5
heterogeneous catalysts. In this contribution, we develop a
Development of silica-supported mesoporous heterogeneous
catalysts for asymmetric catalysis has attracted a great deal of
interest due to their relatively large surface area and pore
volume, and tunable pore dimension and well-defined pore
arrangement. These salient features are beneficial not only to
control the dispersibility of active species but also to adjust the
chiral microenvironment of active centers, showing potential
6
new strategy to assemble a chiral Cp*RhTsDPEN complex
(
Cp* = pentamethyl cyclopentadiene, TsDPEN = 4-methyl-
phenylsulfonyl-1,2-diphenylethylenediamine) within the core
of CSSMSS for the first time, which shows excellent catalytic
efficiency and high recyclability in the asymmetric transfer
hydrogenation of aromatic ketones in aqueous medium. More
importantly, the residual CTAC within CSSMSS materials as
a phase transfer catalyst can enhance greatly the catalytic
performance. In particular, as a new immobilized strategy, the
incorporation of a chiral 1,2-cyclohexanediamine-based Rh(II)
heterogeneous catalyst is also investigated.
1
superiority in stereocontrol performance. Recently, lots of
successful examples, such as nanochannel-type MCM-41/
3
2
SBA-15 and nanocage-type SBA-16 mesoporous materials,
have appeared in the literatures that show highly catalytic
activity and enantioselectivity in various asymmetric reactions.
However their practical applications are still hindered due to the
complicated process of preparation and the low recyclability of
heterogeneous catalysts. Thus, exploitation of novel silica-based
mesoporous materials to overcome these intrinsic disadvantages
plays an important role in industrial applications.
The core–shell structured mesoporous silica spheres with the
Cp*RhTsDPEN functionality in the internal core of silica
spheres, abbreviated as Cp*RhTsDPEN-CSSMSS (3), were
prepared as outlined in Scheme 1. The TsDPEN-functionalized
internal core (2) was obtained via co-condensation of chiral silica
5a
resource 1 and tetraethoxysilane (TOES), in which a mixture
of CTAC and TEA led to high nucleation and the slow growth
of the generated seeds resulted in the mesoporous silica nano-
Core–shell structured mesoporous silica spheres (CSSMSS)
4
as a type of novel silica-based materials show potential
advantage in asymmetric catalysis, in which the core of silica
spheres is convenient to assemble various chiral functionalities while
the shell of silica spheres is beneficial to prevent the leaching of
chiral organometallics, showing potential superiority in recyclability
of heterogeneous catalysts. Furthermore, functionalized CSSMSS
materials can be prepared by a facile process, in which the
co-condensation of inorganosilanes and functionalized
organosilanes followed by continuous growth of inorgano-
silanes is convenient to afford various CSSMSS materials.
More importantly, cetyltrimethylammonium chloride (CTAC)
4a
particles. The continuous growth in the presence of additional
The Key Laboratory of Resource Chemistry of Ministry of Education,
Shanghai Normal University, Shanghai, China.
E-mail: ghliu@shnu.edu.cn, HeXing-Li@shnu.edu.cn;
Tel: +86 21 64321819
w Electronic supplementary information (ESI) available: Experimental
procedures and analytical data for obtained chiral alcohols. See DOI:
1
0.1039/c2cc33512c
Scheme 1 Preparation of the heterogeneous catalysts 3.
This journal is c The Royal Society of Chemistry 2012
7
874 Chem. Commun., 2012, 48, 7874–7876

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Fig. 1 (a) TG/DTA curves of 3, (b) nitrogen adsorption–desorption
isotherms of CSSMSS and 3.
Fig. 2 (a) SEM images, (b) TEM image, and (c) a chemical mapping
of 3 showing the distribution of Si (green) and Rh (yellow).
TOES and CTAC then formed the external shell of the spheres.
5a,b
Finally, direct complexation with [Cp*RhCl2]2
Table 1 summarized catalytic performances of asymmetric
transfer hydrogenation of aromatic ketones without addition
followed by
the trimming of the nanopore via thorough Soxhlet extraction
afforded the catalyst 3 as light red powders (see ESIw in Fig. S1).
The thermal gravimetric (TG/DTA) analysis (Fig. 1a) disclosed
that about 3.1% of the Cp*RhTsDPEN functionality was
anchored within the core of CSSMSS materials by comparison
of TG of pure CSSMSS (see ESIw in Fig. S5), which was nearly
consistent with 4.12 mg (0.040 mmol) of Rh loading per gram for
catalyst detected by inductively coupled plasma (ICP) optical
emission spectrometric analysis. In addition, the 1 : 1 mole ratio
of Rh to S atom was calculated from mass% of S obtained from
elemental analyses (0.13% is equal to 0.040 mmol), suggesting
generation of the single-site Cp*RhTsDPEN functionality within
the core of CSSMSS.
4
of Bu NBr in aqueous medium. In general, excellent conversions
and high enantioselectivities were obtained for all tested aromatic
ketones. Taking acetophenone as an example, the heterogeneous
catalyst 3 gave (S)-1-phenyl-1-ethanol with more than 99%
conversion and 97% ee value, which were higher than those of
Cp*RhTsDPEN without Bu
entry 1), and even comparable to those (99% conversion and
7% ee) of Cp*RhTsDPEN with Bu NBr as a phase transfer
4
NBr (entry 1 versus bracket in
9
4
6c
catalyst. Apparently, the catalyst 3 presented a bifunctional
feature, in which the residual CTAC (ca. 16%, see ESIw in
Fig. S5) acted as
a phase transfer catalyst and the
Cp*RhTsDPEN functionality acts as a chiral catalyst. Obviously
highly catalytic activity could be attributed to the comprehensive
effect of the phase transfer function of CTAC in a biphasic
reaction system and highly dispersive Cp*RhTsDPEN active
centers as confirmed by the TEM chemical mapping. High
enantioselectivity should be due to the fact that the
Cp*RhTsDPEN active centers within the core of CSSMSS
could still keep the original chiral microenvironment, verified
by XPS spectra (see ESIw in Fig. S4), in which the catalyst 3
had nearly the same Rh 3d5/2 electron binding energy as that of
Cp*RhTsDPEN (309.3 eV versus 309.4 eV).
The incorporation of the Cp*RhTsDPEN functionality
within the core of CSSMSS was further confirmed by the
1
3
solid-state NMR spectra. From the C CP MAS NMR spectrum
(
ESIw in Fig. S2), it is clear that the catalyst 3 presented a similar
7
structural arrangement to that of the Cp*RhTsDPEN, proving
the formation of the same well-defined single-site active center as
Cp*RhTsDPEN, in which the typical peaks of TsDPEN moieties
(
16 ppm for Ar–CH
for C H ) and the typical peaks of CpMe moieties (95 ppm for C
5
2
, 70–74 ppm for N–CH–Ph, and 127–134 ppm
6
5
5
29
and 8 ppm CpCH ) could be observed. Meanwhile, the Si CP
3
One important feature of the design of the catalyst 3 is to
obtain high recyclability. In this case, the heterogeneous
MAS NMR spectrum (ESIw in Fig. S3) demonstrated that the
catalyst 3 possessed the inorganosilicate frameworks with
(
3 4
HO)Si(OSi) and Si(OSi) as the main networks due to the
a
3
4
3
Table 1 Asymmetric transfer hydrogenation of aromatic ketones
typical strong Q –Q peaks. The typical T peak suggested that
the Cp*RhTsDPEN functionalities were bonded to the inorga-
3
nosilicate frameworks during the formation of RSi(OSi) (R =
5a,b,8
chiral functionality) as a part of the silica core.
b
Conv. (%)
b
ee (%)
Entry
Ar
As shown in Fig. 1b, the nitrogen sorption measurements
revealed that the catalyst 3 was mesoporous, in which slightly
small values of nanopore size, surface area and pore volume
relative to pure CSSMSS were attributed to Cp*RhTsDPEN
functionalities within the silica core to make the nanopore
1
2
3
4
5
6
7
8
Ph
4-FPh
499(88)
499
499
499
98
499
499
499
499
499
499
499
97 (96)
95
95
93
95
92
95
96
89
4-ClPh
4-BrPh
3-BrPh
4-MePh
4-OMePh
3-OMePh
4-CNPh
3
4-CF Ph
4-NO
2-Naphthyl
5b–d
narrow.
The SEM image revealed clearly that the catalyst 3
presented the uniform size of silica spheres with an average
diameter of around 450 nm (Fig. 2a), while the TEM image
confirmed its core–shell mesostructure in which a silica core
was coated by a silica shell of B50 nm in thickness (Fig. 2b).
In particular, a TEM chemical mapping showed clearly that
the rhodium metallic centers were uniformly distributed within
the core, suggesting that Cp*RhTsDPEN active centers in the
catalyst 3 are highly dispersive within CSSMSS material that
will govern its chiral performance.
9
10
11
93
87
95
2
Ph
1
2
a
Reaction conditions: catalysts (25.00 mg, 1.0 mmol of Rh based on
Na (0.13 g, 0.19 mmol), ketone (0.25 mmol) and
.0 mL water, reaction temperature (40 1C), reaction time (1.0 h).
ICP analysis), HCO
2
2
b
Determined by chiral GC or HPLC analysis (see ESI in Fig. S6).
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7874–7876 7875

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In conclusion, we proposed a facile approach to prepare a
core–shell structured heterogeneous chiral catalyst, which
exhibited excellent catalytic efficiency and high recyclability
in the asymmetric transfer hydrogenation of aromatic ketones
in aqueous medium. In particular, the residual surfactant
within CSSMSS, which acted as a phase transfer catalyst
could enhance greatly the catalytic activity while the presence
of the silica shell could prevent effectively the leaching of
metals. More importantly, this research did also provide a new
immobilized strategy to assemble various chiral functionalities
within CSSMSS material for asymmetric catalysis.
Fig. 3 Reusability of 3 using acetophenone as a substrate.
We are grateful to China NSF (20673072), Shanghai STDF
(
10dj1400103 and 10jc1412300) and Shanghai MEC (12ZZ135
and S30406) for financial support.
Notes and references
1
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(
2
Scheme 2 Asymmetric transfer hydrogenation of the catalyst 4.
´
1
catalyst 3 was recovered easily via centrifugation. As shown
in Fig. 3, the reused catalyst 3 showed an obviously high
recyclability, in which ee values as well as conversions did not
decrease obviously after continuous twelve runs using aceto-
phenone as a substrate (ESIw in Table S1 and Fig. S7). High
recyclability should be due to the fact that the incorporation of
the Cp*RhTsDPEN functionality within the silica core of
CSSMS could decrease greatly the leaching of Rh due to the
protection of the silica shell. An evidence to support the view
was derived from a comparable experiment. In this case, the
material 2 underwent a similar process of preparation to act as
a catalyst. However, after the third recycle, the catalytic
activity decreased greatly. ICP analysis showed that the Rh
amount after the third recycle for 2 was 3.42 mg per gram
2
3
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À1
catalyst while that after the twelfth recycle was 3.98 mg g for
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More importantly, as a new immobilized strategy, assembling
of the various chiral functionalities within CSSMSS materials
was convenient. As shown in Scheme 2, chiral 1,2-cyclohexane-
diamine-derived silica was anchored easily within CSSMSS and
8
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9
Cp*RhTsDACH-CSSMSS (4) was obtained conveniently, in
which the catalyst 4 did also show high recyclability, which could
be run fifteen times without affecting obviously its conversion
and ee value when acetophenone was used as a substrate (ESIw in
Table S2 and Fig. S8).
7
876 Chem. Commun., 2012, 48, 7874–7876
This journal is c The Royal Society of Chemistry 2012